The Simons Array is an experiment located in the Atacama Desert in Chile that will measure the polarization anisotropy of the Cosmic Microwave Background. It consists of three telescopes that house the receivers POLARBEAR-2A, POLARBEAR-2B and POLARBEAR-2C, which will observe the CMB at 90, 150, 220 and 270 GHz with over 22,000 Transition Edge Sensor (TES) bolometers. Each receiver contains a focal plane composed of seven hexagonal arrays of lenslet-coupled sinuous antenna bolometers, with each dichroic pixel containing four TESs. The readout system uses Superconducting Quantum Interference Devices for signal amplification and digital frequency-domain multiplexing with a multiplexing factor of 40. The sensitivity of the Simons Array instruments is governed by the detectors’ noise level and the telescope optical throughput, thus an on-site signal to noise characterization is essential to evaluate the instrument. We present the post-deployment measured readout noise and methods used to improve the noise performance of POLARBEAR-2A detectors, which measure radiation in the 90 and 150 GHz bands.
This conference presentation was prepared for the Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
The Simons Array is a set of three millimeter-wavelength telescopes in the Atacama Desert in northern Chile. It is designed to measure the polarization of the cosmic microwave background caused by density perturbations, gravitational lensing, and primordial gravitational waves. Polarbear-2b (PB-2b) is the receiver that will be mounted onto the Paul Simons Telescope, the second Simons Array telescope. Each pixel in the PB-2b focal plane has a broadband sinuous antenna coupled to transition-edge sensor (TES) bolometers. In all, there are more than 7,500 antenna-coupled TES bolometers which are biased and read out using a digital frequency-domain multiplexing framework. We implement a multiplexing factor of 40 with resonator frequencies ranging from 1.6 MHz to 4.6 MHz. These resonators are connected to superconducting quantum interference device arrays that provide a signal amplification stage. We present Polarbear-2b detector and readout characterization results from in-lab testing that enabled the deployment of PB-2b to Chile in March 2020.
POLARBEAR-2 is a new receiver system, which will be deployed on the Simons Array telescope platform, for the measurement of Cosmic Microwave Background (CMB) polarization. The science goals with POLARBEAR-2 are to characterize the B-mode signal both at degree and sub-degree angular-scales. The degree-scale polarization data can be used for quantitative studies on inflation, such as the reconstruction of the energy scale of inflation. The sub-degree polarization data is an excellent tracer of large-scale structure in the universe, and will lead to precise constraints on the sum of the neutrino masses. In order to achieve these goals, POLARBEAR-2 employs 7588 polarization-sensitive antenna-coupled transition-edge sensor (TES) bolometers on the focal plane cooled to 0.27K with a three-stage Helium sorption refrigerator, which is ~6 times larger array over the current receiver system. The large TES bolometer array is read-out by an upgraded digital frequency-domain multiplexing system capable of multiplexing 40 bolometers through a single superconducting quantum interference device (SQUID).
The first POLARBEAR-2 receiver, POLARBEAR-2A is constructed and the end-to-end testing to evaluate the integrated performance of detector, readout, and optics system is being conducted in the laboratory with various types of test equipments. The POLARBEAR-2A is scheduled to be deployed in 2018 at the Atacama desert in Chile. To further increase measurement sensitivity, two more POLARBEAR-2 type receivers will be deployed soon after the deployment (Simons Array project). The Simons Array will cover four frequency bands at 95GHz, 150GHz, 220GH and 270GHz for better control of the foreground signal. The projected constraints on a tensor-to-scalar ratio (amplitude of inflationary B-mode signal) is σ(r=0.1) = $6.0 \times 10^{-3}$ after foreground removal ($4.0 \times 10^{-3}$ (stat.)), and the sensitivity to the sum of the neutrino masses when combined with DESI spectroscopic galaxy survey data is 40 meV at 1-sigma after foreground removal (19 meV(stat.)).
We will present an overview of the design, assembly and status of the laboratory testing of the POLARBEAR-2A receiver system as well as the Simons Array project overview.
POLARBEAR is a cosmic microwave background (CMB) polarization experiment located in the Atacama desert in Chile. The science goals of the POLARBEAR project are to do a deep search for CMB B-mode polarization created by inflationary gravitational waves, as well as characterize the CMB B-mode signal from gravitational lensing. POLARBEAR-1 started observations in 2012, and the POLARBEAR team has published a series of results from its first two seasons of observations, including the first measurement of a non-zero B-mode polarization angular power spectrum, measured at sub-degree scales where the dominant signal is gravitational lensing of the CMB. The Simons Array expands POLARBEAR to include an additional two telescopes with next-generation POLARBEAR-2 multi-chroic receivers, observing at 95, 150, 220, and 270 GHz.
The POLARBEAR-2A focal plane has 7,588 transition-edge sensor bolometers, read out with frequency-division multiplexing, with 40 frequency channels within the readout bandwidth of 1.5 to 4.5 MHz. The frequency channels are defined by a low-loss lithographed aluminum spiral inductor and interdigitated capacitor in series with each bolometer, creating a resonant frequency for each channel's unique voltage bias and current readout. Characterization of the readout includes measuring resonant peak locations and heights and fitting to a circuit model both above and below the bolometer superconducting transition temperature. This information is used determine the optimal detector bias frequencies and characterize stray impedances which may affect bolometer operation and stability. The detector electrical characterization includes measurements of the transition properties by sweeping in temperature and in voltage bias, measurements of the bolometer saturation power, as well as measuring and removing any biases introduced by the readout circuit. We present results from the characterization, tuning, and operation of the fully integrated focal plane and readout for the first POLARBEAR-2 receiver, POLARBEAR-2A, during its pre-deployment integration run.
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